Rotary cutting tools, such as helical end mills and face mills, are well known in the industry. Conventional helical end mills include those shown on pages 182-216 in Kennametal's Milling Catalog No. 5040, copyright 1995.
One form of helical mill that has been in use for years is a helical mill where the cutting edge of each insert is axially spaced apart from the cutting edge of the next adjacent insert and where the inserts in each helix are staggered such that two or more rows are necessary to produce one complete or “all effective” cutting edge. Such a design is shown by the Kennametal 0 degree Lead-Helical End Mills, as shown on page 188 of the above referenced Milling Catalog. One of the disadvantages of such design is that the lead insert in at least one helix must either be of a different length or offset inward and thus incapable of face milling. This is a result of the need to stagger the inserts in each helix. Consequently, all of the inserts are generally not the same and thus not interchangeable, thus requiring the manufacture and inventory of multiple inserts.
As shown in FIG. 4, a conventional helical end mill 100 includes a tool body 102 and a shank 104. The shank 104 is configured so as to be capable of insertion and securing within a spindle of a milling or other cutting machine (not shown) as is well known in the art. The tool body 102 is a substantially cylindrical body and has a central, longitudinal axis 103. The tool body 102 extends axially from the shank 104 to an end face 106 thereby defining an exterior surface 108 therebetween. The exterior surface 108 of the tool body 102 preferably includes a plurality of helical grooves or flutes 110. It will be appreciated that most any number of helical grooves may be formed in the tool body 102. Each groove 110 is preferably cut into the exterior surface 108 in a helical or spiral manner that extends from the end face 106 to substantially the shank 104.
An insert (not shown) is affixed to each pocket 116 in each groove 110 using a screw (not shown) with a tapered head insertable through a correspondingly tapered hole (not shown) in each insert and then threaded into a threaded hole 118 in the respective pocket 116.
As shown in FIG. 5, each pocket 116 has a bottom support surface 120 that may be at an angle with respect to the axis 103. The pocket 116 also includes an axial support surface 122 and an radial support surface 124 that constitute an axial and radial abutment surfaces, respectively, for the side walls of the insert when mounted in the pocket 116. A corner relief 126 is provided between the bottom support surface 120 and the axial support surface 122. The radial support surface 124 in the vicinity of each pocket 116 may include one or more circular coolant passages 128 for providing coolant to the cutting insert and associated workpiece.
Usually when a helical end mill is in operation, the machining force is exerted against one or more edges of the cutter. The resulting bending moment is resisted by the tool holder which rigidly grips the cutter shank. Ignoring the fact that the direction of the moment changes continually as the cutter revolves, the cutter can be considered to be stressed as a cantilever.
As shown in FIG. 6, simulations have shown that a maximum tensile stress of the pocket 116 during machining operations of approximately 736 MPa (106.76 kpsi) is located at a point 130 proximate the coolant passage 128, resulting in a safety factor for fatigue at 10E6 cycles of about 0.750.
While the matter of stress discussed above is related to avoiding fatigue that would result in tool breakage, no less important is the requirement to minimize tool deflection, in order to improve accuracy and surface finish and to reduce vibration and noise. The bending moment, along with the rotation of the cutter, produces a fully reversed stress condition (alternating tensile and compressive stresses), which is the most devastating condition for fatigue.
Accordingly, there is a need for an improved cutting tool that can overcome the limitations of the known cutting tool, and reduce or eliminate the overall stress of the cutting tool.